The term “prepreg” is generally recognised in the art of fibre-reinforced resin composite materials to describe a blend of continuous high strength fibres (e.g. of carbon fibre, glass fibre, or other known fibre materials) combined with a heat hardenable mixture of resins, in particular thermoset resins, and, where required, hardeners. The fibres may originally be present either as woven fabrics or optionally angled directional fibre arrays which have the resin applied to them in a solid or semi-solid state. The degree and nature of the impregnation of resin, and hardener, into the fibres may vary. The resin, and hardener, may selectively be fully impregnated into the fibres; coated onto one side onto the fibres; partially impregnated into the fibres; or sandwiched between opposed dry fibre layers such that the outer surfaces of the prepreg are free of resin, as disclosed in EP-A-1128958. Such prepregs having opposed dry fibre outer surfaces are easy and clean to cut, stack and react to give a low void content and optimum performance for the fibre and resin materials used in them.
Prepregs can be readily distinguished by those skilled in the art from the manufacture of composite materials made directly from continuous fabrics or discontinuous fibres and liquid resins applied by brush, roller, spray or any other similar method to produce low fibre content “wet lay-up” products. These have an important role in composite manufacture but generally have less than optimum properties with lower fibre contents than are necessary for applications needing the highest possible performance. The liquid resin materials are usually undesirably sticky, difficult to control accurately, and because strongly smelling volatile reactive diluents are often used, require continuous high levels of cleanliness and expensive extraction and recovery facilities in the workplace.
Prepregs can also be readily distinguished by those skilled in the art from the SMC (Sheet Moulding Compounds) or DMC (Dough Moulding Compounds) which are rapid processing materials, in sheet or dough like form, using discontinuous or random fibres and large amounts of mineral fillers combined with fast curing resins. These are cured quickly in relatively thin section between metal moulds to make tough, thin walled cases for many applications including electronic equipment and the like. They are very useful materials in the applications they fulfil but cannot be considered in any physical sense optimised structural composites.
Where necessary to prevent adjacent prepreg surfaces from inadvertently adhering to themselves when presented to the customer for use, or to prevent contamination in the workshop, they may be interleaved on one or both sides with a polyethylene film or alternative release materials.
Typically, prepregs may have nearly or exactly the correct amount of resin in them, matched to the respective fibre content. Once air has been removed from a prepreg assembly by the application of a vacuum, the resin flows under the influence of heat and pressure to fill all the spaces between the fibres. After a heat reaction, called the “curing process” for the resin, the prepreg assembly yields a near or completely void free laminate, the desideratum in a composite fibre-reinforced resin laminate.
Where excess resin is present this must be removed by a variety of techniques well understood by composite material processors to yield void free laminates but in general this is to be avoided where possible as it involves ancillary materials, labour and extra cost.
Such high strength composites have become increasingly used in a wide variety of applications in general industry since their debut in aerospace and some sports goods applications in the early 1970s.
As the applications and volumes of prepregs have multiplied, the fibres and resins from which they have been made have been modified to make them easier and cheaper to buy and use to maximise the performance and volume of products that can beneficially made from them.
However, the composite materials industry has now reached a point where further significant improvements to these fibre and/or resin materials needs to be made so that they may be processed more readily, speedily and cheaply to widen the range of items that can benefit from their properties and can be produced from them more economically.
Aerospace structural composite parts are frequently made from prepregs that are based on resins offering high glass transition temperatures (Tgs) to give large margins of safety should they be exposed to high temperatures, or to very high humidity for long periods of time leading to water saturation of the resins and a lowering of these Tgs but still to acceptable levels. Consequently they tend to use foimulations with a high degree of cross linking which results from using resins with a high reactive group content and a consequent very high heat evolution during cure. This heat evolution must be rigorously controlled by careful processing to avoid excessive temperature rise or damage will result to the composite part.
This level of cross linking with aerospace structural prepregs leads to brittleness which is reduced by the incorporation of significant levels of thermally resistant thermoplastics which in itself leads to high melt viscosities and the need then for high processing pressures. The resins and processes employed tend to be very expensive.
For general structural composites, the current state of the art performance prepregs and composite materials made from them, excluding aerospace primary structural parts, largely consist of glass, carbon and aramid fibres in any required combination, usually impregnated with a blend of solid and liquid Bisphenol A epoxy resins of relatively low molecular weight plus a hardener system which only reacts very slowly at room temperatures giving a storage life of several weeks without significant reaction. This hardener is usually a mixture of finely divided dicyandiamide coupled with a latent urea accelerator. These resin combinations will normally give a substantially full cure after reaction at around 120-130° C. for 1 hour or 12 to 16 hours if cured at 90° C.
These resins are used because they give excellent composite mechanical properties for applications requiring temperature resistance up to the region of 120° C. Most applications in general industry rarely need their best performance above 80° C.-90° C. Current applications include wind turbine blades, leisure and commercial marine, automotive body panels, less critical exterior and most interior aerospace applications, sports goods and the like.
Examples of commercially available prepregs which use lower molecular weight bisphenol and similar epoxy resins and have lower cross link densities than the structural aerospace materials, and typically have a thermal resistance of 100° C.-120° C., include those sold by Hexcel Corporation under the product names M9, M9F, M11, M11.5 and those sold by Gurit (UK) Limited under the product names WE90, WE91, and WT93.
If attempts are made to cure these prepregs quickly, that is at a temperature of around 100° C.-120° C., the temperature range at which the hardener become very reactive, large amounts of heat of reaction are generated in a short time. Unless the composite sections being produced are very thin and the moulds on which they are made conduct this heat away quickly then the composites can reach damaging and even decomposition temperatures.
In thick laminates, i.e. typically having a thickness of greater than 10 mm, in particular greater than 20 mm, made from standard epoxy formulations and glass fibres, temperatures as high as 250° C.-300° C. can easily be achieved. These both damage the composite and often the moulds on which they are made if they are non metallic. The majority of high performance moulds are usually made from epoxy composites themselves and it would be a great advantage if cheaper and lower temperature resistant tools could be used formed from; vinyl ester composites, or even better the much cheaper wet lay-up polyesters composites or the CNC machinable epoxy and polyurethane tooling blocks and pastes.
This damaging temperature rise must be prevented and this is usually achieved by heating the prepreg stack to a level where the reaction just begins and holding it at that temperature, possibly for several hours, whilst a large proportion of the total reaction slowly takes place and the resultant heat is continuously conducted away thus limiting the temperature rise. The reaction is finally completed with the standard cure cycle of around one hour at 120° C. This step is essential to ensure consistency in thick sections and full cure in thin sections. This two step cure process is common practice and for the first lower temperature stage is often referred to as an “intermediate dwell”.
There are particular practical problems producing large components, such as wind turbine blades, from prepregs. A typical composite laminate usually contains areas of different thickness to meet the strength requirements of a given structure. Foams, wood and honeycomb are also often incorporated to form sandwich structures to lighten the construction by separating the fibre reinforced skins with a lower weight core material.
In order to produce such a mixed thickness laminate the method typically used is first to heat the prepreg lay-up to an intermediate dwell temperature to allow the cure reaction to proceed slowly in the thick sections thus allowing the polymerisation to proceed at a rate where the heat produced in the laminate can both flow towards the mould tool and the opposite vacuum bag face. Heat can then be lost through conduction and then either natural or forced convection. Nevertheless, due to the heat releasing nature of the reaction this still usually results in a tolerable temperature increase above the curing temperature, “the exotherm”, in the thicker section during the intermediate dwell but no significant exotherm in the thin sections. Once the exotherm has been controlled in the thicker sections the temperature of the whole lay-up needs to be increased to cure the thin section in a reasonable time.
Without the low temperature intermediate dwell, the rate of heat production would exceed the rate of conduction to the edges of the laminate where it can be lost by normal conduction, convection and radiation. This causes the temperature of the material to rise which in turn leads to a greater rate of reaction producing more heat and a more rapid temperature rise and frequently a large exotherm event. Effectively this may be close to the actual adiabatic temperature rise of which the prepreg is capable. It is not until the reaction rate begins to slow as a significant number of reactive groups have been consumed that the material begins to cool down to the surrounding temperatures.
For example a typical cure for a wind turbine blade using WE90, a DEGBA epoxy prepreg, from Gurit (UK) Limited is likely to have a 1 to 4 hour dwell at 80° C.-90° C. to first control the exotherm, which otherwise might become destructive, followed by a 1 hour further cure at 115-120° C. to ensure full reaction in all areas of the laminate. If the thinner sections of the blade were allowed to remain at 80° C.-90° C. then it would take a further 12 to 16 hours to be certain that full cure had taken place.
Frequently the thickness of the laminates range from 5 to 45 mm for the majority of the blade then increase to 70 mm in some designs to accommodate local bolts or other fixings to attach the turbine blade to the hub assembly. It is clear that heat release must be slow otherwise it would cause an uncontrollable exotherm in the thicker sections.
The design and control of the curing process can become complex. For instance this may need to prevent unwanted exothermic heat flow from the medium thickness areas to the thicker sections, which normally heat up more slowly, triggering early exotherm in them before the reaction has taken place in a controlled manner.
These extended curing cycles are naturally not restricted to wind turbine blades but apply to any thicker section components.
Clearly such cure cycles are both time consuming and severely limit production rates making composites from the current prepregs too expensive for many applications which could benefit from them. Production could be increased by utilising more moulds but these can be very expensive and occupy more factory space resulting in even more cost.
The key factor often limiting the cure speed is the mould tool. For both small volume runs and large parts, such as wind turbines, mould tools tend to be constructed from composite materials. The cost of the tooling materials will increase with the temperature performance. Lower temperature cures are preferred, but are not always possible, as they also help reduce the tool stress and can lead to longer tool life. All such composite tools have a low thermal conductivity and hence exacerbate the exotherm event temperature rise problem.
Typical tooling materials are shown in Table 1.
TABLE 1Typical Tool MaterialsTemperatureResistanceTypeAbove 160° C.Specialist Aerospace Metal ToolsUp to 160° C.High Performance EpoxyUp to 130° C.Performance Epoxy.Up to 110° C.High Performance Vinyl Ester.Up to 80° C.High Performance Polyester.Up to 70° C.Epoxy modelling pastes, High performance Epoxytooling blocksUp to 50° C.Low performance tooling block and modelling pastes
A hypothetical idealised prepreg would exhibit not all of the following combination of properties:
1. Possess a reasonable storage life to enable manufacture, testing, packing, shipment and customer shop floor use time. Ideally this is around three months at ambient temperatures, but may be as short as one week in certain circumstances. The useable life of all prepregs may be increased by cold storage.2. Absence of strong smelling or significantly volatile materials during normal storage, handling, cutting, lay-up or curing conditions.3. No adverse reactions with water or carbon dioxide at any stage of prepreg storage or use.4. Easy to cut neatly, cleanly and readily to any desired shape by recognised methods.5. Good drape and tack characteristics for applying into a mould or any other equipment used for forming.6. Capability of reaction to give the final optimum cured product at a temperature no higher than 130° C. but desirably as low as 60° C. in one hour or less at the cure temperature.7. Heat of reaction evolution should not permit the total maximum temperature achieved in any thickness composite moulding to exceed 160° C. and most desirably 100° C. or even lower.8. If the heat of reaction can be reduced significantly the prepreg can be cured more rapidly and possess a “snap cure” characteristic to further reduce the cure time to provide further productivity benefits. In this specification, the term “snap cure” means the curing of a prepreg resin in a period of at most 45 minutes, preferably less than 30 minutes and more preferably less than 15 minutes, after reaching the cure onset temperature.9. Ability to be cured by ultra violet and/or visible radiation.10. Cured properties to satisfy the end composite material requirements fully and consistently, ideally matching those of current prepregs to avoid the need to redesign components to accommodate new products.
The usual approach in currently known lower exotherm epoxy prepregs to manufacture thick laminate components is to formulate the opposite of a snap curing material—that is materials are formulated to have a broader heat release curve to try to reduce reactivity closer to the cure onset temperature. This provides a temperature window for the component manufacturer to programme an intermediate dwell within the tolerance capability of their heating system to control the initial heat release by holding at a temperature where the reaction proceeds at a slow enough rate to avoid a damaging out of control exotherm. This approach leads to undesirably long cure cycles.
Most unsaturated resins, such as vinyl or polyester resins cured by latent free radical reaction possess a snap cure characteristic. U.S. Pat. No. 6,436,856B1 discloses such a vinyl ester prepreg. The vinyl ester resin is supplied diluted in styrene monomer to both reduce the overall cost of the composition and the starting viscosity to allow simple machinery to be used to impregnate the fibre reinforcement. The composition also contains magnesium oxide to increase the viscosity of the resin after impregnation from a low viscosity liquid into a prepreg viscosity often referred to as a B-staging process, in the same manner as is used to manufacture polyester and vinyl ester SMC and dough moulding compounds.
Such a resin has a high unsaturation per Kg and is estimated to have a heat of polymerisation of 350-450 KJ/Kg in the examples given in U.S. Pat. No. 6,436,856B1. On reaching the activation temperature of the peroxide catalyst the prepreg begins to cure very quickly and self accelerate leading to a very large exotherm event in a thick laminate.
Inhibitors can be added to absorb the free radicals generated by the latent curing agent but these tend to only delay the onset of the reaction as they work by scavenging the free radicals generated as the curing agent decomposes to produce free radical curing agents. Once the inhibitor has been consumed or simply is ineffective due to the volume of free radical generated at the decomposition temperature, the polymerisation reaction continues at a rapid rate and the heat rise from the polymerisation further self accelerates the generation of free radicals leading to an uncontrollable exotherm event. Providing an excess of inhibitor or lower amounts of free radical initiators also proves ineffective as this can cause the resin to be under cured.
As such the practical use of these prepregs has been limited to the manufacture and rapid cure of thin laminates where the heat can be loss by conduction into the mould tool and radiation and convection from any exposed surfaces.
Latent hardeners can be more easily selected to control the heat release during cure of epoxy resins. For both mechanical performance and processing reasons epoxy resins have to a large extent been the matrix resins of choice for making most high performance composites. It would be highly desirable to have prepregs that have both a long shelf life at room temperature to remove the need for refrigerated storage, and transport. So far this has proved difficult. Typically a 90° C. curing prepreg would have a shelf life of 8 weeks at 20° C. and a low temperature curing 50-60° C. system a shelf life of 1-3 days at 20° C. and these prepregs are transported and stored in temperature controlled and sub-ambient conditions.
The majority of these epoxy systems have been based on the readily available Bisphenol A (4,4′ dihydroxyphenyl 2,2 propane) series. This is a homologous series of essentially diglycidyl ethers. They range from the crystalline virtually pure monomer, through flowing liquid resins to semi solids, solids and ultimately to very high molecular weight polymers with almost no epoxy content.
Other epoxy resins based on Bisphenol F (4,4′ dihydroxyphenyl methane) and oligomers of this as well as those based on higher molecular weight polyfunctional novolac resins have also been used. Much the same reasoning below applies to these epoxy resins as well.
It is standard practice to blend liquid and low molecular weight solid versions from the range to yield mixtures which are fluid enough at safe temperatures when containing the curing agent to enable good impregnation of fabrics and fibres and casting of films, and flexible and tacky enough as prepregs for good processing at shop temperatures, and with good viscosity control for processing into high quality laminates.
In some cases no tack and low flow are required and then a slightly higher proportion of solid resin will be used.
The following calculations demonstrate the current exotherm problem clearly.
Glycidyl epoxy groups of the type found in these resins usually have a heat of polymerisation in the region of 98.4 KJ per mole (23,500 cals per mole).
To increase the final thermal performance it is necessary in most structural aerospace applications to use an epoxy resin with a rigid backbone and a higher epoxy content to increase the final cross link density. These resins with a high epoxy content result in high heat of polymerisation.
Current “state of the art” lower exotherm prepregs are made with a blend of difunctional liquid and solid epoxy resins and have a lower final thermal performance. If there is too much liquid in the formulation they are too tacky to handle and do not have sufficient body to maintain the fibres in place. If there is too much solid resin then they become rigid and brittle. The ratio of liquid to solid epoxy resins in most such prepregs usually falls in the range of 60:40 to 40:60 by weight.
Examples of such lower exotherm epoxy prepregs are sold by Hexcel Corporation under the product names M9, M9F, M11, M11.5 and those sold by Gurit (UK) Limited under the product names WE90, WE91, and WT93 and would have an average heat of polymerisation in the range 230 to 375 KJ/Kg when measured using Differential Scanning Calorimetry (DSC).
All of these epoxy prepregs require an indeterminate dwell to allow the cure to first take place at a slow reaction rate to prevent a damaging exotherm in thicker laminates.
To improve productivity and reduce the risk of exotherm damage for new components an increasing trend is to attempt to model the cure cycle dwell times to optimise the curing processing, but even this often leads to only small percentage reductions in the overall cure times. Each newly configured composite material part then requires a new remodelling and optimising process.
To avoid the need for this simulation it would be highly desirable to reduce the exotherm so that any heat generated would be insufficient to damage the mould tool or other materials within the laminate stack to allow a simpler, more tolerant, cure to be used that would negate the need for an inteimmediate dwell step.
One current approach in prepregs to reducing the curing exotherm is therefore to have a more gradual heat release after the temperature of curing initiation (T onset) has been attained, to give an opportunity to control heat release with a more gradual reaction rate.
Thus there is a major need in the composite materials industry to provide improved, more versatile prepregs that possess a reasonable storage life, are free from strong smelling or significantly volatile materials, have no adverse reactions during storage and use, have good drape and tack for the desired application, have good mechanical and thermal resistance, and can be cured quickly without a damaging exotherm event.
A prepreg with these characteristics would be a major advance for most composite fabrication applications and it is an aim of this invention to provide such a prepreg.
It is accordingly an aim of this invention to provide a prepreg, a method of manufacturing prepregs and a method of processing prepregs which at least partially overcomes at least some of these significant disadvantages of the existing fibre and/or resin materials currently used to manufacture prepregs.